The Gravity Recovery and Climate Experiment (GRACE) mission revolutionized the understanding of mass transport in the Earth system, enabling for the first time the recovery of a global time variable gravity field. GRACE mission from 2002 to 2017 enabled scientists answering questions about the water mass transfers in the Earth surface, including ice-sheet mass loss especially in Greenland and Antarctica, the discharge and accumulation of water in land, and the increase in the Ocean mass, among other key variables. In combination with radar altimetry, gravimetry missions also enables to monitor some essential aspects of the Earth energy cycle such as the Energy Imbalance (EEI), which is responsible for the accumulation of heat in the climate system. Thanks to the GRACE Follow-On (GRACE FO) mission, the satellite gravimetry record has been extended to decadal time scales enabling the analysis of longer term changes in the water-energy cycle . The next generation of gravity missions MAGIC should enable to push further the spatial resolution and the time resolution in the recovery of the time variable gravity field leading to more precise and more frequent estimates of the components of the water and energy cycle of the Earth. But it is only with technical breakthrough that a step forward in accuracy can be achieved opening the way for a global and repetitive remote sensing of essential climate variables such as the AMOC circulation, the regional ice melt, the deep ocean warming, and the time variability of the EEI. In this presentation we propose 1) to review the different aspects of climate change that are monitored by space gravimetry , 2) to recall recent developments achieved with currently available GRACE and GRACE-FO records 3) to present the expected scientific benefits from MAGIC and discuss the challenges from a climate perspective, for future gravity missions beyond the next generation.

In the next decade, it is expected that the next generation gravity missions will provide improved both spatial and temporal resolution. Specifically, it is expected to have sub-monthly (weekly) temporal resolution and a spatial resolution better than 100 km. These improved capabilities will open new opportunities for exploiting gravity data, and specifically water storage measurements, for advanced hydrological applications.
In this presentation we will show three preparatory activities that have been carried out to be ready to exploit such new wealth of data, and specifically related to (1) runoff estimation, (2) drought monitoring, and (3) precipitation estimation.
In the STREAM and STREAMRIDE projects funded by ESA, we have developed a new approach exploiting satellite precipitation, soil moisture and water storage data (from GRACE and GRACE-FO) for estimating runoff and river discharge at continental and global scale. The modelled runoff has been found in good agreement with river discharge estimation and provides comparable performance with respect to more complex land surface modelling.
Monitoring drought from space can be implemented by using satellite soil moisture measurements that however are able to sense just a thin soil layer. Gravity data can provide information on subsurface water content thus offering new capabilities. Particularly, an improved spatial resolution will give the opportunity to perform such analysis also for smaller river basins, like the Mediterranean area that is a "hot spot" for the study of climate change.
In several projects (funded by ESA and EUMETSAT), we have developed the SM2RAIN algorithm for estimating precipitation from satellite soil moisture observations. Global rainfall datasets based on this algorithm has been produced and freely available. Current gravity measurements have a temporal resolution (monthly) too coarse to be used in this framework. However, gravity data at weekly (or better) resolution can be of high value for providing enhanced observation of soil water storage and, hence, improved precipitation estimates through SM2RAIN algorithm.

Next Generation Gravity Missions are expected to enhance our knowledge of mass transport processes in the Earth system, establishing their products applicable to new scientific fields and serving societal needs. Compared to the current situation (GRACE Follow-On), a significant step forward to increase spatial and temporal resolution can only be achieved by new mission concepts, complemented by improved instrumentation and tailored processing strategies.

In 2015 consolidated user needs for sustained mass transport observation from space have been derived under the umbrella of IUGG. They formed the basis for follow-up documents such as the Earth Explorer 9 proposal e.motion2 and the Earth Explorer 10 proposal MOBILE, the Final Report of the NASA/ESA Interagency Gravity Science Working Group, and were evaluated and enhanced for the Mission Requirement Document (MRD) of the joint NASA/ESA mission concept Mass change And Geosciences International Constellation (MAGIC).

In this contribution, we will give an overview of the user requirements and needs, with special focus on quantum space gravimetry mission concepts and the sustained observation of climate signals. We will also analyze the error budget of current gravity satellite missions in order to identify the biggest error contributors, and discuss potential future improvements regarding instrumentation, satellite constellations and processing techniques to meet these requirements. A specific focus will be given to quantum sensors for accelerometry, which are from an instrument point of view the biggest error contributor, complemented by temporal aliasing errors resulting from tidal and non-tidal background models. We will try to quantify the relative contributions of these errors to the total gravity mission performance, in order to identify the benefit of improved measurement technologies based on quantum sensors.

In mission concepts that are focusing on temporal changes of the gravity field reflecting mass transport processes in the Earth system, quantum accelerometers based on cold-atom techniques are used for measuring non-conservative forces in satellite-to-satellite tracking concepts. The main advantage of quantum instruments is their close to flat error spectrum, thus reducing high-amplitude long-wavelength errors of classical electrostatic accelerometers. We will quantify the impact and the potential of these new sensors for future gravity observation from space, and their potential for an improved monitoring of climate-induced processes in the Earth system.

Climate Change is one of the major challenges of the age with implications on every aspect of society, economy, and ecology. The first step to act on the impact of the changing climatic environment is the identification of measured effects, such as the reduction of shelf ice or the resulting sea level rise. Those effects are monitored on a local (i.e. stationary ground-based or on moving platforms, such as planes and cars) and global (i.e. satellite-based) scale with increasing sensitivity over the past years.
GRACE (Gravity Recovery And Climate Experiment), GRACE-Follow On, and GOCE (Gravity field and steady-state Ocean Circulation Explorer) have been deployed for this very purpose. They successfully mapped the gravitational field of the Earth and allowed the monitoring of changes, being indicative of the impact of climate change.
The resolution of the gravitational field, and thereby the efficacy of the height-measurement, depends on the knowledge of the orbit, the residual accelerations on the satellite(s), and the accuracy of the link between them. Quantum technologies are prone to support the development of more precise next generation Earth observation missions.
As such, frequency stabilization of the link between two satellites in a GRACE-FO constellation, benefits from state-of-the-art optical frequency references and optical links. The combination of these optical technologies allows for more precise laser ranging measurements due to the shorter wavelength with respect to microwave or radio frequency links. In addition, the optical link to ground and the involved pointing accuracy has increased, due to the proposed requirements for ESAs next generation gravity mission (NGGM) program.
Another option to increase sensitivity and monitoring properties is cold atom interferometry. Atom interferometers deploy an ensemble of condensed atoms, a so-called Bose Einstein condensate, to perform matter-wave interferometry. Interferometry has always been a precise tool to measure differential changes. With atoms as test masses, accelerations can be detected directly. As an added benefit, ultra-cold atoms are levitated in an ultra-low vacuum environment. This enables measurements of accelerations without interference due to friction. As such, atom interferometry is a quantum technology that could be deployed in gradiometry and gravity missions alike. Experiments with cold atom sensors have been performed on planes and in vehicles, probing local fields. In combination with the miniaturization, ruggedization, and automation necessary to operate cold and condensed atoms in scientific missions, such as QUANTUS, MAIUS, CAL, and BECCAL, their usage in space based gravity missions, either in a GOCE or GRACE-FO configuration, are envisaged.
This enables quantum technologies to be deployed for the monitoring of climate change and induced changes on critical environmental areas, such as the ice shelfs, the oceans, and land masses. In addition, it is crucial to monitor the impact of possible counter measures. Only with constant monitoring is it possible to judge the efficacy of taken measures and their possible impact on other systems.
With the unique constellation of friction-less, acceleration sensitive quantum sensors, other areas in environmental monitoring can be addressed. This enables early warning systems for events such as earthquakes, volcanic eruptions, and floods. All of which can have devastating results for population and environment.
In addition to monitoring the impact of climate change, developments in quantum based magnetometry can detect changes in Earths environment. Furthermore, magnetometers have proven efficient in the organization of transport by detection of surrounding vehicles. In combination with improved navigational systems, this enables the enhancement of efficiency of global transport, reducing emissions, a key ingredient in fighting climate change. Similar to gravity missions, navigational systems can be improved by deploying optical quantum technologies.
In conclusion, it can be stated, that quantum technologies can be deployed to detect the impact of climate change, support counter measures, inform decisions, and monitor developments. Due to their properties, the systems are prone to improve our view on the world and support the fight against the impacts of climate change.

The potential for application of cold atom interferometry in inertial navigation, geophysics, or tests of fundamental physics motivate the interest for the development of space-borne matter-wave accelerometers and gradiometers. The concept relies on a high common mode rejection and a longer interaction time due to micro gravity environment to achieve unprecedented sensitivity. After the potential shown in ground laboratories, the focus is now on the technical challenges of a realistic space mission implementation.
After GOCE's success and while planning the Next Generation Gravity Mission (MAGIC), Thales Alenia Space looks forward to maintaining its leading position in supporting the roadmap of the next generation gravitational sensors.

The separation of atomic energy levels provides a previously unobtainable accuracy and precision in metrology, with an SI traceable reference to frequency and wavelength [1]. This achievable performance is widely exploited in laser cooled atomic samples, enabling long interactions time that are not available in thermal vapour samples without additional interactions.

Laser cooled atomic samples are now widely implemented in state-of-the-art atomic clocks and interferometers, opening a range of applications including navigation, gravity sensing and acceleration. As a result of the improved performance that laser cooling can provide to these applications, significant efforts have been made in the miniaturisation of cold-atom instruments to facilitate the needs of field-deployable quantum technologies.

In recent years our group has focussed on the micro-fabrication of optical components to aid the miniaturisation of cold-atom sensors to the chip-scale through the development of the grating magneto-optical trap (GMOT). While such technology provides a means to a reduced optical apparatus, a number of critical components required for laser cooling remain unsuitable for in-field applications.

A core element that remains essential for the portability of cold-atom instruments is a miniaturised, self-contained and passively pumped vacuum system, forming the enclosure for a sample of atoms cooled by laser light [2]. Additionally, the footprint of the laser light optical system requires reduction through the innovation of novel wavelength reference for laser stability and atomic probing. However, for cold-atom instruments to reach their full potential for market exploitation and application range, these components must be micro-fabricated, to enable mass production and a reduced cost of manufacturing [3].

This talk will cover an introduction to the field of cold atom based atomic sensors and why these platforms lie at the heart of quantum technology endeavours. We will highlight our recent progress towards a fully micro-fabricated cold-atom sensor platform [4]. We will discuss our on-going research on the micro-fabrication of novel atomic vapour cells for laser locking and laser cooling applications to ultimately bring cold-atom instrumentation down to the chip-scale.

Additionally, we will discuss key advancements from the wider community that well address the miniaturisation and in-field deployment of cold-atom sensors that could enable such technology being transferred to space borne sensors.

References
1. J. Kitching, Chip-scale atomic devices, Applied Physics Reviews 5, 031302 (2018)
2. J. P. McGilligan, et. al., Laser cooling in a chip-scale platform, Applied Physics Letters 117, 054001 (2020)
3. J. A. Rushton, et. al., Contributed Review: The feasibility of a fully miniaturized magneto-optical trap for portable ultracold quantum technology, Review of Scientific Instruments 85, 121501 (2014)
4. A. Bregazzi, et al. A simple imaging solution for chip-scale laser cooling, Appl. Phys. Lett 119, 184002 (2021)